3-dimensional imprint tool

ABSTRACT

The invention is a method for the fabrication of an imprint tool master. The process begins with a metallic substrate. A layer of photoresist is placed onto the metallic substrate and a image pattern mask is then aligned to the mask. The mask pattern has opaque portions that block exposure light and “open” or transparent portions which transmit exposure light. The photoresist layer is then exposed to light transmitted through the “open” portions of the first image pattern mask and the mask is then removed. A second layer of photoresist then can be placed onto the first photoresist layer and a second image pattern mask may be placed on the second layer of photoresist. The second layer of photoresist is exposed to light, as before, and the second mask removed. The photoresist layers are developed simultaneously to produce a multi-level master mandrel upon which a conductive film is formed. A tool master can now be formed onto the conductive film. An imprint tool is then produced from the tool master. In one embodiment, nickel is electroplated onto the tool master to produce a three-dimensional imprint tool.

[0001] The United States Government has rights in this inventionpursuant to Contract No. DE-AC04-94AL85000 between the United StatesDepartment of Energy and Sandia Corporation for the operation of SandiaNational Laboratories.

BACKGROUND OF THE INVENTION

[0002] 1. Field of the Invention

[0003] The present invention relates to circuit board fabrication andmore specifically, it relates to a method for fabricating threedimensional imprint tools.

[0004] 2. Description of Related Art

[0005] Previous attempts by others in making imprint tools have sufferedlargely in two areas. Firstly, the process required two plating stepsand the adhesion of the nickel layers was insufficient. Secondly, theylacked control of the height/depth and the cross-sectional areas of thefeatures were not accurately reproduces in the thermoset laminatematerial. These are both serious issues affecting the replication andfunctionality of the final product.

[0006] One method of making an imprint tool included etching a coppersubstrate to make a master. A pattern is chemically milled into thecopper sheet and a nickel tool is electroformed from the master.Problems with this method include lack of control of the depth of theetch over large areas and the typical rounding of the pattern sidewallfrom etching. These are both serious problems since etching deepercauses more rounding and greater distortion of the pattern.

[0007] A second method uses a two-step plating process to form the traceand stud/via. This process starts with patterning the traces on a nickelsheet and plating the traces with nickel. This is followed by applyinganother photoresist layer and patterning the studs. This method sufferedfrom lack of adhesion between the nickel layers, trace/stud, as well asthe substrate. It is difficult aligning the second pattern over thefirst plated layer and it was difficult controlling the height of thestuds during the second plating.

[0008] It is desirable to provide methods for making an imprint toolthat are superior to the methods described above in eliminating adhesionproblems, in controlling the feature height and in providing superiortrace and stud cross-sectional areas.

SUMMARY OF THE INVENTION

[0009] It is an object of the present invention to provide a process forfabricating three dimensional imprint tools.

[0010] It is another object of the invention to eliminate adhesionissues by electroforming the trace/stud as an integral component.

[0011] Another object of the invention is to provide a method forcontrolling feature height and for overcoming overplating or geometrycurrent density issues.

[0012] Still another object of the invention is to provide a method forfabricating three dimensional imprint tools having trace and studcross-sectional areas that are superior over trace and studcross-sectional areas produced with etching or by the prior art two stepprocess.

[0013] To maintain consistency in the fabricated tool, the presentinvention provides a method for fabricating a tool Master from which theembossing tool of the present invention are made. The present methodallows robustness to be built into the tool Master by making it anydesired thickness. The use of two layers of dry film photoresist isunique to this invention. This method provides a tool designer theopportunity to incorporate any combination of embossing features intothe design of the tool, including, but not limited to, features as smallas a few microns to as much as 50 micron or more. Furthermore, thepresent invention also affords the flexibility to design tools havingfeatures of varying geometry “stacked” one on top of another feature,thus providing for a multi-level imprinting tool.

[0014] The use of liquid photoresist can also be used in this method, aswell as the utilization of dry film photoresist. The dry filmphotoresist is produced with a very controlled manufacturing processthat affords control over the thickness or height of the features thatare grown.

[0015] Imprint technology can be used in all sectors of government andindustry currently using conventional printed circuit technology. Thepresent invention is useful in high density circuit fabrication becausethe via or plated through-hole is made concurrently with the traceformation. This allows closer spacing of traces and eliminates the needfor large angular rings (pads) around the vias. This imprint technologyeliminates several wet processing steps used in current technology,resulting in increased production quantity and decreased production costcompared to existing imprint technology.

[0016] The instant invention, therefore, is a method for the fabricationof an imprint tool Master and a method for fabricating an embossing toolusing the tool Master. The process begins with a sheet of stainlesssteel or titanium having a thickness of about 0.090 inches. Stainlesssteel and titanium, of course, are illustrative materials only. Otherchoices of materials and material sheet thicknesses are possible so longas the choice is electrically conductive and exhibits a reasonablestiffness-to-weight ratio. A dry photoresist film is first laminatedonto the stainless steel or titanium substrate sheet. (Liquidphotoresist can also be used in this method, as an alternate to the useof dry film photoresist; however, the dry film photoresist contributesto a very controlled manufacturing process that affords control over thethickness or height of the features that are grown.) A mask comprising anegative trace image of a desired pattern, a circuit pattern forexample, is then placed on the dry film photoresist and the film exposedto light through the mask openings. The mask and the Mylar cover sheetare then removed and a second dry film photoresist is laminated onto thefirst dry photoresist film if a multi-layer structure is desired. Insuch cases, a second mask comprising a second negative trace image isaligned over the second dry film photoresist and the second film exposedas was the first.

[0017] The next step in this process is to use standard developingmethods to develop both of the exposed dry film photoresist layers atthe same time. This results in the removal of the unexposed portions ofthe dry film photoresist layers, leaving only the exposed portions onthe stainless steel or titanium substrate sheet. The surface of thissheet and the developed portions of the photoresist are then coveredwith a copper film in order to form a conductive adhesion layer and athick plate of nickel electroplated onto the copper layer. This nickellayer is typically about 0.030 inches to about 0.040 inches in thick butthicker or thinner layers are possible, and may be desirable, given aparticular end-use circumstance.

[0018] In a final step, the electroplated thick nickel layer is removedfrom the stainless steel or titanium sheet, is cleaned to remove copperand photoresist residue, and is chemically passivated for furtherprocessing.

[0019] The cleaned nickel plate now embodies a tool master, from whichan embossing tools can be fabricated.

BRIEF DESCRIPTION OF THE DRAWINGS

[0020]FIG. 1A shows the first step in an embodiment of the presentmethod where a sheet of stainless steel or titanium is provided.

[0021]FIG. 1B illustrates the lamination of photoresist film onto thesheet.

[0022]FIG. 1C shows the placement of a negative trace image of a desiredcircuit pattern on the photoresist.

[0023]FIG. 1D shows the removal of the negative trace image from thephotoresist.

[0024]FIG. 2A illustrates the lamination of a second photoresist filmonto the first photoresist film.

[0025]FIG. 2B shows the step of aligning a second negative trace imageonto the second film of photoresist.

[0026]FIG. 2C shows the removal of the second negative trace image fromthe second film of photoresist.

[0027]FIG. 2D illustrates the step of developing both of the exposedphotoresist films.

[0028]FIG. 3A shows the stainless steel or titanium sheet, upon which isthe developed exposed portions of both photoresist layers, upon whichcopper is deposited.

[0029]FIG. 3B shows a thick plate of nickel electroplated onto thecopper layer of FIG. 3A.

[0030]FIG. 3C shows the removal of the thick plate of nickel from thestainless steel or titanium sheet to form the tool master.

[0031]FIG. 3D shows the step of electroplating the tool master withnickel to produce an embossing tool.

[0032]FIG. 3E shows the final embossing tool separated from the toolmaster.

DETAILED DESCRIPTION OF THE INVENTION

[0033] The present invention is a process for fabricating threedimensional imprint tools. This process eliminates any adhesion issuesby electroforming the trace/stud as an integral component. The featureheight is controlled by plating the tool master in reverse order, whichremoves any overplating or geometry current density issues. Thecross-sectional area of the trace and studs are superior over trace andstuds produced with etching or by the prior art two step process.

[0034] General Description

[0035] The invention is generally a method for the fabrication of animprint tool master. The process begins with a metallic substrate. Alayer of photoresist is placed onto the metallic substrate and a maskpattern, hereinafter a circuit pattern is described for illustrativepurposes, is then aligned to the mask. The circuit pattern mask hasopaque portions that block exposure light and “open” ortransparent/translucent portions that transmit exposure light. Thephotoresist layer is then exposed to light transmitted through the“open” or transparent/translucent portions of the first circuit patternmask and the circuit pattern mask is then removed. A second layer ofphotoresist is then placed onto the first photoresist layer and a secondcircuit pattern mask is placed on the second layer of photoresist. Thesecond layer of photoresist is then exposed to light and the secondcircuit pattern mask is removed. The photoresist layers are developedsimultaneously to produce a master mandrel upon which a conductive filmis formed. A tool master is then formed onto the conductive film. Animprint tool is produced from the tool master. In one embodiment, nickelis electroplated onto the tool master to produce a three-dimensionalimprint tool.

[0036] Specific Description

[0037] An embodiment of the steps of the invention are described withreference to FIGS. 1A through 1D and continues with reference to FIGS.2A through 2D and is completed in FIGS. 3A through 3E.

[0038] As required, detailed embodiments of the present invention aredisclosed herein. However, it is to be understood that the disclosedembodiments are merely exemplary of the present invention which may beembodied in various systems. Therefore, specific details disclosedherein are not to be interpreted as limiting, but rather as a basis forthe claims and as a representative basis for teaching one skilled in theart to variously practice the present invention.

[0039] Referring to FIG. 1A, the process begins with a sheet 10 ofstainless steel or titanium, for example. This sheet should be about0.090 inches thick. A dry photoresist film 12, usually about 0.002inches thick, is then laminated onto the sheet 10, as shown in FIG. 1B.This dry film photoresist is known as “out of the box dry film.” Dryphotoresist film usually includes a Mylar cover sheet 14. As discussedabove, liquid photoresist can also be used in this method, as analternate to the use of dry film photoresist; however, the dry filmphotoresist contributes to a very controlled manufacturing process thataffords control over the thickness or height of the features that aregrown.

[0040] In the next step, as shown in FIG. 1C, a negative trace image 16of a desired circuit pattern is placed on the cover sheet 14 of the dryfilm photoresist 12. The desired circuit pattern will coincide with theclear or transparent/translucent portions of the negative trace image16. The dry film photoresist 12 is then subjected to light exposurethrough the negative trace image 16. Referring to FIG. 1D, the negativetrace image 16 and the Mylar cover sheet 14 are removed. The clear Mylarcover sheet 14 is about 0.001 inches thick. The remaining materialcomprises stainless steel or titanium sheet 10, which carries dryphotoresist film 12, which includes exposed portions 12′.

[0041] A second dry film photoresist 18 (out of the box dry film) with asecond clear Mylar cover sheet 20 (about 0.001 inches thick) is thenlaminated onto the dry photoresist film 12 (which includes exposedportions 12′), as shown in FIG. 2A. Dry films 12 and 18 are here thesame material. They are distinguished from one another in order todistinguish their use in separate process steps and to suggest that,although the use of identical materials is disclosed it is not necessarythat they be identical. Furthermore, the thicknesses of the twophotoresist films likely will be different in order to provide fordiffering feature heights in the finished tool.

[0042] The next step of aligning a second negative trace image 22 ontothe dry film photoresist 18 is illustrated in FIG. 2B. The desired studpattern should coincide with the clear or transparent/translucentportions of the negative trace image 22. Using a standard circuit boardexposure light source, the second dry film photoresist layer 18 is thenexposed. As shown in FIG. 2C, the negative trace image 22 and the clearMylar cover sheet 20 then are removed. The remaining material comprisesstainless steel or titanium sheet 10, which carries dry photoresist film12, which carries dry photoresist layer 18, which includes exposedportions 18′.

[0043] As shown in FIG. 2D, the next step is to use standard developingmethods to develop both of the exposed dry film photoresist layers 12and 18 at the same time. This results in the removal of the unexposedportions of the dry film photoresist layers 12 and 18, leaving only theexposed portions 18′ on 12′which is on sheet 10.

[0044]FIG. 3A illustrates sheet 10, also referred to as a Mastermandrel, upon which is the developed exposed portion 12′ upon which isthe developed exposed portions 18′. The surface of this sheet 10 and thedeveloped portions 12′ and 18′ are then covered with a thin electricallyconductive film 24 through a particle vapor deposition (PVD) process,although any other coating process which would provide a thin,continuous layer of material would be equally effective. Such methodsinclude, but are not limited to, sputtering and chemical vapordeposition metal or metalloids, and dipping, spraying, or spin coatingmaterials solutions or suspensions and only require that the coatingprocess not damage the photoresist layer and result in a continuous,conductive layer.

[0045] As disclosed herein, film 24 is copper, but could be any similarmaterial including any of the Transition series of metal listed in NewIUPAC Group Numbers 4-12 of the Period Table of elements, and alloysthereof, certain of the metals and metalloids of Groups 13 and 14,conductive polymers, as well as colloidal suspensions and paints of anyof the foregoing materials. This film is necessary to enable adherenceof a first thick metal layer 26 which is deposited in a subsequent step.In the present invention, layer 26 is nickel but, as before, it could beany similar metal selected from the Transition series of metal listed inNew IUPAC Group Numbers 4-12 of the Period Table of elements, as well astin, or any alloy thereof providing that layer 26 is dissimilar to film24.

[0046] Referring to FIG. 3B, a thick layer 26 of nickel is electroplatedonto the film 24 in this subsequent step. The nickel layer of thepresent invention is here about 0.030 inches to about 0.040 inchesthick. The use of nickel and the thickness of this layer is, however,illustrative only. Layer 26 could be nickel or any similar metalselected from the Transition series of metal listed in New IUPAC GroupNumbers 4-12 of the Period Table of elements, as well as tin, or anyalloy thereof, depending upon the use desired (if for instance a hardermaterial was required). Thinner or thicker layers are also possible, andmay be even be desirable, depending upon the particular circumstance ofthe intended end-use.

[0047] The final step in fabricating a tool master 30 is illustrated inFIG. 3C where the thick layer 26 of nickel is removed from the stainlesssteel or titanium sheet (substrate base) 10. After layer 26 is cleanedto remove copper and photoresist residue, it now describes a tool master30 from which embossing tools can be fabricated.

[0048] The step of fabricating an embossing tool 40 from the tool master30 is illustrated schematically in FIG. 3D. This final step of formingembossing tool 40 by electroplating a desired thickness of a secondthick metal layer 28 onto the tool master 30. Again, as before, layer 28could be nickel or any similar metal selected from the Transition seriesof metal listed in New IUPAC Group Numbers 4-12 of the Period Table ofelements, as well as tin, or any alloy thereof. Before electroformingembossing tool 40, however, nickel tool master 30 is dipped into adilute aqueous solution of sodium dichromate (30 to 60 seconds in 1 gm/lof water) to passivate the plating surface. The choice of passivatingsolutions, of course, will depend upon the metal chosen to form toolmaster 30.

[0049] Typically, the tool 40 is electroplated onto the tool master 30until it reaches a thickness of between 0.010 inches and 0.013 inches.However, thickness greater than these are possible, and may even bedesirable. Fabrication of such thick sheets is limited only by theplating apparatus and the plating time necessary to achieve the desiredthickness. As shown in FIG. 3E, the final tool 40 is then separated fromthe nickel tool master 30, is cleaned, and is then ready for use as anembossing tool.

[0050] The foregoing description of the invention has been presented forpurposes of illustration and description and is not intended to beexhaustive or to limit the invention to the precise form disclosed. Manymodifications and variations are possible in light of the aboveteaching. The embodiments were chosen and described to best explain theprinciples of the invention and its practical application to therebyenable others skilled in the art to best use the invention in variousembodiments and with various modifications suited to the particular usecontemplated. The scope of the invention is to be defined by thefollowing claims.

What is claimed is:
 1. A method for fabricating an imprint tool master,comprising: providing a metallic substrate; forming a first photoresistlayer onto said substrate; aligning an image pattern mask onto saidphotoresist layer, wherein said pattern mask has portions that blockexposure light and portions which transmit exposure light; exposing saidphotoresist layer with a first exposure of exposure light; removing saidfirst pattern mask; developing said first photoresist layer to produce amaster mandrel; forming a conductive film onto said master mandrel; andforming a tool master on said conductive film.
 2. The method of claim 1,further comprising forming an embossing tool from said tool master. 3.The method of claim 2, wherein the step of forming an embossing toolcomprises: passivating said tool master; electroplating a metal layeronto said tool master; and separating said metal layer from said toolmaster to provide said embossing tool.
 4. The method of claim 3, whereinthe step of electroplating a metal layer includes electroplating a metalselected from the group consisting of the Transition series of metallisted in New IUPAC Group Numbers 4-12 of the Period Table of elements,tin, and any alloy thereof.
 5. The method of claim 3, wherein the stepof electroplating includes electroplating nickel.
 6. The method of claim1, wherein the step of providing a metallic substrate comprisesproviding a stainless steel substrate.
 7. The method of claim 6, whereinthe step of providing a stainless steel substrate comprises providing astainless steel substrate that is about 0.090 inches thick.
 8. Themethod of claim 1, wherein the step of providing a metallic substratecomprises providing a titanium substrate.
 9. The method of claim 8,wherein the step of providing a titanium substrate comprises providing atitanium substrate that is about 0.090 inches thick.
 10. The method ofclaim 1, wherein the step of forming a first photoresist layer onto saidsubstrate comprises forming a first dry film photoresist layer on saidsubstrate.
 11. The method of claim 1, wherein the step of forming afirst photoresist layer onto said substrate comprises forming a firstwet photoresist layer onto said substrate.
 12. The method of claim 1,wherein the step of aligning an image pattern mask comprises aligning apositive trace image pattern mask.
 13. The method of claim 1, whereinthe step of aligning an image pattern mask comprises aligning a negativetrace image pattern mask.
 14. The method of claim 1, wherein the step offorming a second photoresist layer comprises forming a second dry filmphotoresist layer.
 15. The method of claim 1, wherein the step offorming a second photoresist layer comprises forming a second wetphotoresist layer.
 16. A method for fabricating an imprint tool master,comprising: providing a metallic substrate; forming a first photoresistlayer onto said substrate; aligning an image pattern mask onto saidphotoresist layer, wherein said pattern mask has first portions thatblock exposure light and first portions which transmit exposure light;exposing said photoresist layer with a first exposure of exposure light;removing said first pattern mask; forming a second photoresist layeronto said first photoresist layer; aligning a second pattern mask of asecond desired pattern onto said second photoresist layer, wherein saidsecond pattern mask has second portions that block exposure light andsecond portions which transmit exposure light; exposing said secondphotoresist layer with a second exposure of said exposure lighttransmitted through said second portions; removing said second patternmask; developing both said first photoresist layer and said secondphotoresist layer to produce a master mandrel; forming a conductive filmonto said master mandrel; and forming a tool master on said conductivefilm.
 17. The method of claim 16, further comprising forming anembossing tool from said tool master.
 18. The method of claim 17,wherein the step of forming a tool comprises: passivating said toolmaster; electroplating a metal layer onto said tool master; andseparating said metal from said tool master.
 19. The method of claim 18,wherein the step of electroplating a metal layer includes electroplatingThe method of claim 3, wherein the step of electroplating a metal layerincludes electroplating a metal selected from the group consisting ofthe Transition series of metal listed in New IUPAC Group Numbers 4-12 ofthe Period Table of elements, tin, and any alloy thereof.
 20. The methodof claim 18, wherein the step of electroplating a metal layer includeselectroplating nickel.
 21. The method of claim 16, wherein the step ofproviding a metallic substrate comprises providing a stainless steelsubstrate.
 22. The method of claim 21, wherein the step of providing astainless steel substrate comprises providing a stainless steelsubstrate that is about 0.090 inches thick.
 23. The method of claim 16,wherein the step of providing a metallic substrate comprises providing atitanium substrate.
 24. The method of claim 23, wherein the step ofproviding a titanium substrate comprises providing a titanium substratethat is about 0.090 inches thick.
 25. The method of claim 16, whereinthe step of forming a first photoresist layer onto said substratecomprises forming a first dry film photoresist layer on said substrate.26. The method of claim 16, wherein the step of forming a firstphotoresist layer onto said substrate comprises forming a first wetphotoresist layer onto said substrate.
 27. The method of claim 16,wherein the step of aligning an image pattern mask comprises aligning apositive trace image pattern mask.
 28. The method of claim 16, whereinthe step of aligning an image pattern mask comprises aligning a negativetrace image pattern mask.
 29. The method of claim 16, wherein the stepof forming a second photoresist layer comprises forming a second dryfilm photoresist layer.
 30. The method of claim 16, wherein the step offorming a second photoresist layer comprises forming a second wetphotoresist layer.
 31. The method of claim 16, wherein the step ofaligning a second pattern mask comprises aligning a second positivetrace image pattern mask.
 32. The method of claim 16, wherein the stepof aligning a second pattern mask comprises aligning a second negativetrace image pattern mask.
 33. The method of claim 16, wherein the stepof forming a conductive film comprises forming a conductive film byparticle vapor deposition.
 34. The method of claim 16, wherein the stepof forming a conductive film comprises forming a copper film.
 35. A 3dimensional embossing tool, comprising: an electroformed metal sheethaving a print positive electroformed image of an embossing pattern,said pattern including a plurality of structures having variablegeometries and minimum feature dimensions as small as about 0.0005inches.
 36. The embossing tool of claim 35, wherein said plurality ofstructures include multi-layered structures.
 37. The embossing tool ofclaim 36, wherein each of said layers has a thickness ranging from about0.0005 inch to about 0.01 inch.
 38. A tool master for forming a 3dimensional embossing tool, comprising: an electroformed metal sheethaving a print negative image of an embossing structure formed into saidelectroformed sheet, said sheet formed by electroplating a thickness ofa metal over a print positive image comprising a metallic coatedphotoresist.